1. Print Statement vs. Print Function

In Python 2, the print keyword is a statement and does not require parentheses. In Python 3, print is a built-in function and must be called with parentheses.

Python 2 Example:

print "Hello, World!"
print("Hello, World!")  # Also works

Python 3 Example:

print("Hello, World!")
# print "Hello, World!"  # This line would cause a SyntaxError

2. Input Function Behavior

Python 2 has input() and raw_input(). The input() function evaluates user input as Python code, while raw_input() returns a string. Python 3 only has input(), which always returns user input as a string.

Python 2 Example:

user_data = raw_input("Enter text: ")  # Returns a string
result = input("Enter an expression: ")  # Evaluates the expression

Python 3 Example:

user_data = input("Enter text: ")  # Always returns a string

3. Integer Types

Python 2 distinguishes between int (32-bit) and long (unlimited precision) integers. Python 3 consolidates both into a single int type with unlimited precision.

Python 2 Example:

print(type(42))        # <type 'int'>
print(type(2**100))    # <type 'long'>

Python 3 Example:

print(type(42))        # <class 'int'>
print(type(2**100))    # <class 'int'>

4. Divition Operator

In Python 2, the / operator performs floor division (integer division) when both operands are integers. True division requires at least one float operand. In Python 3, / always performs true division (returning a float), while // is used for floor division.

Python 2 Example:

print(7 / 2)    # Output: 3
print(7.0 / 2)  # Output: 3.5

Python 3 Example:

print(7 / 2)    # Output: 3.5
print(7 // 2)   # Output: 3

5. Inequality Operators

Python 2 supports two inequality operators: != and <>. Python 3 only supports !=.

Python 2 Example:

print(3 != 4)  # True
print(3 <> 4)  # True

Python 3 Example:

print(3 != 4)  # True
# print(3 <> 4) # This line would cause a SyntaxError

6. Range and Xrange Functions

Python 2's range() returns a list, while xrange() returns a memory-efficient iterator. Python 3's range() returns an iterator-like object, and xrange() is removed.

Python 2 Example:

print(range(3))    # [0, 1, 2]
print(xrange(3))   # xrange(3)

Python 3 Example:

print(range(3))    # range(0, 3)
# print(xrange(3)) # NameError: name 'xrange' is not defined

7. Exception Handling Syntax

Python 2 uses a comma to capture an exception instance, while Python 3 uses the as keyword.

Python 2 Syntax:

try:
    # ...
except ValueError, err:
    print(err)

Python 3 Syntax:

try:
    # ...
except ValueError as err:
    print(err)

8. String Literals and Encoding

Python 2 source code uses ASCII as the default encoding for string literals unless explicitly declared otherwise. Python 3 source code uses UTF-8 by default.

Python 2 Example (Header Declaration Required):

# -*- coding: utf-8 -*-

9. Boolean Constants

In Python 3, True and False are reserved keywords and cannot be reassigned. In Python 2, they are built-in names that can be overridden (though this is not recommended).

10. String Formatting

Python 3 introduced f-strings, a concise way to embed expressions inside string literals. This feature is not available in Python 2.

Python 3 Example:

name = "Alice"
print(f"Hello, {name}!")  # Output: Hello, Alice!

11. Package Structure

In Python 2, an __init__.py file is mandatory for a directory to be recognized as a package. In Python 3, it's optional (though often still used).

12. String Module Constants

Python 2's string.letters, string.lowercase, and string.uppercase constants, wich were locale-dependent, are replaced in Python 3 with the ASCII-specific string.ascii_letters, string.ascii_lowercase, and string.ascii_uppercase.

Python 3 Usage:

import string
print(string.ascii_letters)  # abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ

13. Class Definition

Python 2 has both "old-style" (classic) and "new-style" classes, distinguished by inheriting from object. Python 3 only has "new-style" classes, which inherit from object by default.

Python 2 Classic Class:

class LegacyClass:
    pass  # Old-style class

Python 2 New-style and Python 3 Class (All are new-style):

class ModernClass:
    pass  # Implicitly inherits from object in Python 3

class ModernClassExplicit(object):
    pass  # Explicit inheritance